1. Field of the Invention
The field of the invention is that of optical devices that can be used for contactlessly measuring the orientation of an object in space. There are various possible fields of application but the main application is detecting the posture of an aircraft pilot helmet so that an image can thus be projected onto his visor so that it is exactly superposed on the external landscape or so that various systems of the aircraft can be subjugated to his gaze. The precision sought for such systems is of the order of one milliradian.
2. Description of the Prior Art
There are various optical techniques that can be used for measuring orientation on a helmet. In general, noticeable elements are installed on the helmet and are pinpointed by a system of cameras. The position of the images of these noticeable elements makes it possible through calculation to determine the orientation of the helmet.
These elements may be passive; or active. Passive elements are illuminated by an external source. For this purpose, retroreflective corner cubes can be used as these make it possible to reduce the problems of parasitic light caused by solar illumination. All that is required is for the optical emission and reception members to be arranged on the same axis.
Active elements are generally light-emitting diodes. The cameras have a fixed focus and therefore a depth of field which is of necessity limited.
This technique has a certain number of disadvantages. The quality of the image of each point imaged on the detector is dependent on the position of the helmet and on its orientation, thus limiting the precision of the system if a significant measurement volume or a substantial range of rotation is to be covered.